[RISCV] Teach .option arch to support experimental extensions.

Previously .option arch denied extenions are not belongs to RISC-V features. But
experimental features have experimental- prefix, so .option arch can not
serve for experimental extension.
This patch uses the features of extensions to identify extension
existance.

Author: Yeting Kuo

llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp

@@ -2824,8 +2824,9 @@ bool RISCVAsmParser::parseDirectiveOption() {
         break;
       }
 
-      auto Ext = llvm::lower_bound(RISCVFeatureKV, Arch);
-      if (Ext == std::end(RISCVFeatureKV) || StringRef(Ext->Key) != Arch ||
+      std::string &&Feature = RISCVISAInfo::getTargetFeatureForExtension(Arch);
+      auto Ext = llvm::lower_bound(RISCVFeatureKV, Feature);
+      if (Ext == std::end(RISCVFeatureKV) || StringRef(Ext->Key) != Feature ||
           !RISCVISAInfo::isSupportedExtension(Arch)) {
         if (isDigit(Arch.back()))
           return Error(
@@ -2834,7 +2835,7 @@ bool RISCVAsmParser::parseDirectiveOption() {
         return Error(Loc, "unknown extension feature");
       }
 
-      Args.emplace_back(Type, Ext->Key);
+      Args.emplace_back(Type, Arch.str());
 
       if (Type == RISCVOptionArchArgType::Plus) {
         FeatureBitset OldFeatureBits = STI->getFeatureBits();

llvm/test/MC/RISCV/option-arch.s

@@ -1,7 +1,7 @@
 # RUN: llvm-mc -triple riscv32 -show-encoding < %s \
 # RUN:   | FileCheck -check-prefixes=CHECK %s
 # RUN: llvm-mc -triple riscv32 -filetype=obj < %s \
-# RUN:   | llvm-objdump  --triple=riscv32 --mattr=+c,+m,+a,+f,+zba -d -M no-aliases - \
+# RUN:   | llvm-objdump  --triple=riscv32 --mattr=+c,+m,+a,+f,+zba,+experimental-zicfiss -d -M no-aliases - \
 # RUN:   | FileCheck -check-prefixes=CHECK-INST %s
 
 # Test '.option arch, +' and '.option arch, -' directive
@@ -78,6 +78,13 @@ lr.w t0, (t1)
 # CHECK: encoding: [0xb3,0x22,0x73,0x20]
 sh1add t0, t1, t2
 
+# Test experimental extension
+# CHECK: .option arch, +zicfiss
+.option arch, +zicfiss
+# CHECK-INST: sspopchk ra
+# CHECK: encoding: [0x73,0xc0,0xc0,0xcd]
+sspopchk ra
+
 # Test '.option arch, <arch-string>' directive
 # CHECK: .option arch, rv32i2p1_m2p0_a2p1_c2p0
 .option arch, rv32i2p1_m2p0_a2p1_c2p0